In addition, former Justice Minister Adnan Addoum and the commander of the Presidential Guards, Brig. Gen. Mustafa Hamdan, were summoned to report to the U.N. investigation, the officials said.

Chief U.N. investigator Detlev Mehlis wants to question the five men, who are known to be pro-Syria, the officials said, speaking on condition of anonymity owing to the investigation's sensitivity.

The detainees were: Maj. Gen. Jamil Sayyed, the former chief of General Security; Maj. Gen. Ali Hajj, the former director general of the Internal Security Forces; and Brig. Gen. Raymond Azar, the former director general of military intelligence.

The detentions were the police's first major move since Hariri and 20 people were killed in a massive bomb in Beirut on Feb. 14. Under Lebanese law, those held for questioning can be detained for up to four days, but the order may be extended. Afterward, the detainee is either released, freed on bail or formally arrested.

The detainees and the general have previously been questionned by Mehlis, the German prosecutor who is leading the U.N. Security Council-mandated investigation into the assassination.

Mehlis also asked for former legislator Nasser Qandil, a staunch supporter of Syria, to be detained for questioning, the officials said. But when police went to Qandil's house in Beirut, he was not there and his wife said he was in Syria, the officials said.

Addoum and the three former security chiefs had stepped down in April as calls mounted for their dismissal from Lebanese politicians opposed to Syria. Addoum was both prosecutor general and justice minister at the time of Hariri's assassination. A preliminary U.N. inquiry into the killing accused the authorities of tampering with evidence.

Many Lebanese blamed Hariri's assassination on Syria and pro-Syrian elements of their government. Syria and its Lebanese allies denied any involvement.

The killing of Hariri, who was seen as being quietly opposed to Syria's role in Lebanese affairs, was a turning point in Lebanese history. It provoked massive demonstrations against Syria and the pro-Syrian government in Lebanon, and it intensified international pressure on Syria to leave its western neighbor. The pressure forced the resignation of the government and led to Syria's withdrawing its troops from Lebanon in late April, ending a 29-year military presence in the country.

Hariri, who was prime minister for 10 of the past 12 years, presided over Lebanon's reconstruction after the 1975-90 civil war. He was also a billionaire businessman.

Many Lebanese have expressed fear about the consequences of the results of Mehlis' investigation, particularly if it blames people connected to Syria, which retains considerable influence in Lebanon. Last week the United Nations accused Syria of refusing to turn over documents to the investigation and ignoring requests for interviews.

Since Hariri's killing, there have been a series of smaller bombs have exploded in commercial centers and cars, killing several people, including two anti-Syrian activists. The attacks are thought to be linked to tension between those who favor a Syrian role in Lebanon and those who oppose it, the AP reports.